1. Field of the Invention
This invention relates to a half ester of an organic polyol made by reacting polyols with anhydrides; to a thermosettable composition including the half ester of an organic polyol, an anhydride, an epoxide, and a catalyst; and to a thermoset product made from such thermosettable composition.
The thermosettable compositions of the present invention are useful in various applications such as casting, potting, and encapsulation, such as electrical and electronics applications, and composites. The thermoset products made from the thermosettable compositions of the present invention have improved mechanical performances, especially toughness and mechanical strength, while maintaining high thermal resistance.
2. Description of Background and Related Art
Some thermosetting resins are known to have good toughness and good mechanical properties, some thermosetting resins are known to have good thermal resistance, and some thermosetting resins are known to have good processability. However, heretofore, a thermosetting resin exhibiting each and every one of the above characteristics to the extent that such thermosetting resin can be prepared in large scale, and used in high performance applications under significant stress and moisture exposure, has not been made.
There is a need for thermosetting resins with improved mechanical properties (e.g. higher toughness and mechanical strength) while maintaining good thermal resistance and good processability of the resin. High toughness and high mechanical strength leads to less cracking and high mechanical integrity, reducing the number of defects and improving the reliability and life time of the end products. High thermal resistance enables high operation temperature. Low formulation viscosity improves processability and enables high filler loading.
Heretofore, those skilled in the art have attempted to improve the properties of thermosetting resins by adding various additives, such as flexibilizers and toughening agents, to the thermosetting resin. However, the use of conventional flexibilizers such as linear polyols in the thermosetting resin only leads to a moderate improvement in toughness; and significantly reduces the thermal stability of the thermoset because of the resulting low (less than 80° C.) glass transition temperature. The use of conventional toughening agents in the thermosetting resin leads to processing issues because of the high viscosity of the resulting formulation, and because of the complexity of the phase-separation process.
For example, the use of conventional toughening agents such as liquid rubbers, core-shell particles and thermoplastic polymers, in thermosetting resins increases the toughness of the thermosetting resin at the cost of adversely affecting some other properties of the thermosetting resin such as glass transition temperature, mechanical properties, viscosity, etc. It is difficult to maintain the glass transition temperature and mechanical strength when liquid rubbers and thermoplastic polymers are employed to improve toughness; and due to the nature of core-shell particles, it is hard to fully disperse these particles into a thermosetting resin such as an epoxy resin without additional treatments.
Still, there are various prior art technologies that have been used in an attempt to improve the mechanical properties (e.g. higher toughness and mechanical strength) of thermosetting compositions. It would desirable, however, to formulate a half-ester with a thermosetting resin, such as an epoxy resin, to produce a low viscosity formulation suitable for casting, potting, encapsulation, and impregnation processes; wherein the final thermoset product resulting from said formulation displays superior mechanical and thermal properties.
U.S. Pat. No. 4,313,859 discloses a polymerizable liquid mixture of (a) a half ester characterized by the following empirical formula: wherein n is a number having an average value of about 1.5 to less than about 4, m is equal to the free valence of R less the average value of n, R is the hydroxyl-free residue of an organic polyol which contained from 2 to 4, inclusive, hydroxyl groups, OH, in formula (I) (b) maleic anhydride, (c) an epoxide containing two or more 1,2-epoxide radicals, (d) an ethylenically unsaturated monomer which forms a liquid homogeneous mixture with the half ester, maleic anhydride and epoxide, and (e) a basic compound.
However, the composition disclosed in U.S. Pat. No. 4,313,859 contains an ethylenically unsaturated monomer; and does not contain a “saturated” “cyclic” anhydride. “Cyclic” anhydrides contain an anhydride functionality within a ring. “Saturated” anhydrides contain no ethylenic unsaturation but may contain aromatic ring, optionally substituted or partially substituted, and/or optionally hydrogenated or partially hydrogenated such as phthalic anhydride and derivatives. “Unsaturated” anhydrides, on the other hand, contain ethylenic unsaturation that becomes incorporated into the backbone of the ester chain after reaction. Maleic anhydride is an example of an unsaturated anhydride. U.S. Pat. No. 4,313,859 describes the manufacture of a vinyl ester resin which is a different process than a process that uses a cycloaliphatic diol such as 1,3-cyclohexane dimethanol and 1,4-cyclohexane dimethanol which are not described in U.S. Pat. No. 4,313,859.
U.S. Pat. No. 4,497,945 discloses that a tough epoxy polymer is formed by pre-reacting a poly (oxypropylene) diol or triol with a chemical excess of an anhydride curing agent for epoxy resins. The resulting diester-diacid disclosed in U.S. Pat. No. 4,497,945 is then polymerized with an epichlorohydrin-bisphenol A epoxy resin. U.S. Pat. No. 4,497,945 discloses the use of imidizole catalysts for the reactions.
Pre-reacting a poly (oxypropylene) diol or triol having a molecular weight (MW) of about 1000 to about 3000 is different from using a lower molecular weight resin (for example, MW<1000, Dow UNOXOL Diol™ has an average molecular weight of about 144) as disclosed in U.S. Pat. No. 4,497,945. U.S. Pat. No. 4,497,945 also does not disclose the use of a lower MW alcohol or the use of an injection molding (IM) process.
WO 9731965 discloses a process for making high-performance polyetherester resins and thermosets. WO 9731965 discloses that a polyether polyol reacts with a dicarboxylic acid or anhydride in the presence of an insertion catalyst to produce an acid-terminated polyetherester resin. This resin reacts with a primary diol or a diepoxy compound to give a chain-extended polyether ester resin that can be cured with a vinyl monomer to produce a high-performance polyether ester thermoset. Properties of the resulting thermosets rival or exceed those of the more expensive high-performance iso and vinyl ester resins.
The polyether polyol disclosed in WO 9731965 has an average hydroxyl functionality of about 2 to about 8, and a number average molecular weight of about 250 to about 25 000, preferably of about 1000 to 4000, which differs from polyether polyols that have an average molecular weight <1000, preferably <250. The prior art polyol is specifically a polyether polyol. The polyetherester resins are further advanced with a primary diol or an epoxy resin in order to reduce the acid number. The resulting resins are cured (cross-linked) with a vinyl monomer in the presence of a radical initiator. The technology disclosed in WO 9731965 differs from the use of a half-ester to react with an epoxy resin to form the thermoset. WO 9731965 discloses the use of a chain extender such as 1,4-cyclohexane-dimethanol, which is very easy to become solid (crystallization) during processing. WO 9731965 does not disclose the use of a mixture of 1,3-cyclohexane dimethanol and 1,4-cyclohexane dimethanol which is very stable and is a liquid at ambient temperature. This is very important to maintain acceptable processability of the final formulation.
U.S. Patent Application Publication No. 1994274949A discloses a process for the preparation of a linear tertiary aliphatic carboxyl functional polyester resin by reacting a) at least one compound A′ comprising one monofunctional primary- or secondary hydroxyl group and/or at least one compound A″ comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group and b) at least one aromatic or cycloaliphatic dicarboxylic acid compound B comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof and c) at least one diol compound C comprising two aliphatic hydroxyl groups which may each independently be a primary or a secondary hydroxyl group and d) at least one dihydroxymonocarboxylic acid compound D comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl, the molar ratio of compounds A′:A″:B:C:D being M:N:X+Y+1:X:Y, wherein M+N=2, X ranges from 2 to 8 and Y ranges from 2-N to 8, at a temperature of from 100° C. to 240° C., until essentially all the nontertiary carboxyl groups as initially present in the reaction mixture have been reacted.
The starting materials and process used to make the acid functional polyester as disclosed in U.S. Patent Application Publication No. 1994274949A is a completely different chemistry than using a half ester. U.S. Patent Application Publication No. 1994274949A teaches using a monofunctional primary or secondary hydroxyl group.
U.S. Patent Application Publication No. 1995516144A discloses a method for producing esters of hydroxyl terminated polybutadiene, including the steps of: (a) providing an anhydride; (b) reacting a hydroxyl terminated polybutadiene with the anhydride to form a carboxyl terminated polybutadiene derivative; (c) reacting the derivative with an epoxide. Also a curable composition, including: (a) an ester of hydroxy terminated polybutadiene; (b) a copolymerizable ethylenically unsaturated monomer; and (c) a drier. The chemistry related to an ester of a hydroxyl terminated polybutadiene differs from a reaction product comprising a carboxyl terminated polyether ester.
U.S. Patent Application Publication No. 1995391329A discloses a curable suspensions of an epoxy resin formulation comprising a) a storage-stable suspension of an epoxy resin and a toughener suspended therein which contains no groups that react with a curable epoxy resin system, b) dicyandiamide, a polycarboxylic acid, a polycarboxylic anhydride, a polyamine, a polyaminoamide, an amino group-containing adduct of an amine and a polyepoxide, a polyol or a catalytically curing hardener, and, as optional components, c) a curing catalyst, conventional fillers or reinforcing materials, are particularly suitable for use as casting resins, laminating resins or adhesives.
The starting materials used in U.S. Patent Application Publication No. 1995391329A are blended together without any reaction between them. U.S. Patent Application Publication No. 1995391329A does not disclose an anhydride which is pre-reacted with polyols. In U.S. Patent Application Publication No. 1995391329A, a core/shell polymer toughener is incorporated into the system to improve the toughness.
Most conventional toughening agents, such as rubbers, core-shell particles, thermoplastic block polymers, have a high molecular weight to initiate suitable toughening mechanisms. The high molecular weight, however, has an undesirable effect on the viscosity of the uncured thermoset formulation. Because of the high viscosity, these formulations cannot be used to prepare compositions that need to diffuse into small cavities, and they also cannot be used with conventional processes (e.g., because of limitations of the pumps). To lower the viscosity of these conventional formulations, the processing temperature of the formulation is increased, but this also leads to shortened pot life.
It would be desirable to provide a formulation that uses polyols such as cycloaliphatic polyols that affords improved properties. Accordingly, there is a need to develop a curable composition comprising a half ester of an organic polyol; wherein such the half-resins have a low molecular weight and/or a low viscosity cycloaliphatic polyol and anhydride, which impart greater processability, maintain excellent toughening properties without a dramatic reduction of thermal stability.
Prior art technology use polyether polyols in formulations which do not afford good mechanical properties while maintaining thermal resistance. Accordingly, there is a need in the industry for a curable polyol modified composition having an increase in tensile strength, flexural strength, and especially the impact strength while without significantly decreasing glass transition temperature compared with known polyol modified compositions.